There are proposed systems to which communication techniques are applied that use high frequency signals in a high frequency region such as a microwave region at 1 to 30 GHz or a millimeter wave region at 30 to 300 GHz. Examples of such systems include data communication systems and radar systems.
Some high frequency circuits that use high frequency signals have a circuit configuration in which a high frequency element such as a MMIC (monolithic microwave integrated circuit) is mounted on a circuit board having a waveguide line such as a microstrip line. On the high frequency element mounting surface, not only the waveguide line but also various element circuits used for the system are formed. These element circuits and high frequency signals radiated through the waveguide line may be affected by each other. Thus, an antenna board having an antenna may be disposed on a back surface of the circuit board. There is a technique for directly connecting the circuit board and the antenna board via a solder bump. This technique is described, for example, in Japanese Unexamined Patent Publication JP-A 2004-254068.
In the high frequency module described in JP-A 2004-254068, an input-output port of a circuit board and an input-output port of an antenna board are arranged on mutually opposing surfaces of the boards. In such a high frequency module, in the case where a plurality of openings of waveguides are formed, the size of the circuit board increases.
An object of the invention is to provide a circuit board whose size can be reduced, a high frequency module and a radar apparatus.